I have described elsewere a tranferportal. this basically takes a craft at aircraft type speeds and converts it to orbital velocity and in addition with ' a kick up its behind sends it into space. My origional concept was an orbiting passive energy exchanger which would transfer the energy of an incoming vehicle to an outward going one. This apart from a slight couple would not suffer from momentum problems. The trouble is that there is a limit to how fast you can accelerate a man. What we would be looking at is a device of the order 100 miles in lengh. I next considered a series of orbiting ' coils ' acting as an electromagnetic accelerator using solar power and building up currents in super conducting coils. This has momentum problems reaction ) and requires massive instulations.I am now thinking small. If one does not need to accelerate aman merely a load ( parts to assemble a craft, fuel, stores ) then the path can be reduced the ' coils ' attached to one another by ' cables ' as they are close one must have a series of opposing polarity which can work against the cables. One has a series of push pull devices a harness ( retrievable ) around the load will switch polarity of the load's coil where necessary plus there would be 90 degrr coils for stability. The load can be brought up in two ways: a direct sold state rocket which needs only to bring the load up to orbit i can then be stationary held before falling back and swept up by the portal to orbital velocity, or the gated sram ( NASA ) waste bin can make regular trips ( passangers paying money ) between USA and East Asia at the end of the atmospere at speeds of 3000-4000 mph. If aload is detached and perpedicular thrust rockets fired the loads horizontal velocity can be converted to a vetical one which with a little help would reach orbit height etc. Thus a transferable space station could be constructed ( constructers hosed in station but brought up by con ventional rocket. Since this spaceship/station does not have to take of from earth but move from earth orbit to mars orbit it can be made quie roomy.At the sme time a duplicate ship can be built BUT WTH A CONTRA ROTATION. In this way the momentum probem can be overcome (this is all greatly simplified ). U sinf fuel stores built up the ship can then take off for Mars and enter into Mars orbit where It could act as the base for men ferried down to the surface. If there are further ' coils ' orbiting earth at greater distance then the portal can be con verted ito a space cannon shootin g supplies to Mars ( note there will be momentum problems to be overcome.THe cannon can also be used to send fuel for the return trip. In all cases a retro rocket system must be fitted to drop into Mars orbit. Using the complementry retrogade space ship a premanent space shuttle system Earth/Mars Mars/Earth can be set up.This is also appicable to Moon Venus Jupiter's Moons etc.The Portal can be used to build bigger portals.Yes I know the Physics Stinks its to inspire ideas that's all

It sounds great in principle. But with all such complicated schemes there are many technical detail problems that make it practically unworkable.

Take for example the "momentum problem" that you rather hand-wave away. Loss of velocity changes its orbit, which means both its altitude, period, etc. This makes coordinating captures and shots very difficult (orbital intercepts are tricky even when the targets are moving around).

Maybe someday when we have a better grasp on space flight and situations of regular transfers from place to place. One where your idea of an orbital mass driver might be useful is to make it two way. Use it to decellerate and capture an incoming "packet" to regain orbital energy in preparation to give it away to an outbound one.